Leap Year Explained

Rotation math problem on a chalkboard
Why do we need leap years?

By Pamela Gourley-Delaney, Ph.D.

LeapFrog Learning Expert

As a Senior Learning Designer for LeapFrog, Pamela applies her teaching and research experience to the design of engaging learning experiences for children of all ages. She taught ages 1 to 12 on three different continents and her research work examines learning through children’s media, toys, books, and parent education. She stays informed with the help of her two playful children. Pamela earned her doctorate in Education at the University of California, Davis.

How long is a year?

Many people would confidently answer “365 days, of course!” But did you know that it actually takes 365 days, 5 hours, 48 minutes and 45 seconds for Earth to circle around the sun? This means that a 365-day calendar year doesn’t count the almost 6 extra hours every year!

Luckily, there’s a solution. You may have guessed it: Leap Year!

We have an extra 6 hours each year. So every 4 years, these extra hours add up to 24 hours, which conveniently add up to one whole day. To compensate for these extra hours, we celebrate leap year just about every four years by adding a leap day at the end of our shortest month on February 29.

Exceptions to the rule

Wait! You might realize that the numbers don’t seem to work out completely. Since our calendar is only short 5 hours, 48 minutes and 45 seconds in most years, adding a full 24 hours every 4 years slightly overcompensates for the actual difference between the calendar year and the time it takes to circle around the sun. To correct for this error, we sometimes skip leap years based on these two important rules:

  • Most century years (e.g. 1700, 1800, 1900) are NOT leap years.
  • However, when a century year is divisible by 400 (e.g. 1600, 2000, 2400), it IS a leap year.

Does that sound too complicated? Well, since the next exception is not until the year 2400, we can pretty safely use the “divisible by 4” rule for the foreseeable future.